Validate commands that can have only a single key

[kaustubh-nair]

Commands like autocomplete can support only a single key, due to restrictions by the urwid_readline package. We want to ensure that the user does not specify multiple keys, on commands like these.

[kaustubh-nair]

@zulipbot add "enhancement"

[zulipbot]

Hello @zulip/server-hotkeys members, this issue was labeled with the "area: hotkeys" label, so you may want to check it out!